Devices employing low temperature superconductors must be refrigerated to bring the superconducting material below the critical temperature at which the material becomes superconducting. For most superconductors this critical temperature is extremely low, often in the vicinity of the temperature of liquid helium or hydrogen. Thus, the difference between room temperature and the temperature of the superconducting device can be very large. As a result of this large temperature difference the energy lost through even the smallest thermally conductive area can be significant.
Current is usually supplied to a superconducting device from outside the refrigerated device. Normally conductive leads, often carrying very high currents, must bridge the room temperature environment of the power supply and the very low temperature environment of the superconducting device. Normal conductors of electricity--especially those designed for high current loads--are also excellent thermal conductors. Consequently, when normal conducting leads are connected to the refrigerated superconducting device, they can place a significant load on the refrigerating equipment for the superconducting device. The added power required to refrigerate the device can significantly degrade the total power efficiency of the system.
In many superconductor applications, for example in the superconducting magnets used in nuclear magnetic resonance diagnostic machines and particle accelerators, a current is only applied to the device during the charge and discharge of the device. Since the time devoted to charging and discharging the device can be very small in proportion to the time when the device need not be electrically connected to the exterior (in a standby mode), it is common practice to disconnect the electric leads into the device throughout the standby period.
Unfortunately, the mechanical joints in detachable leads reduce the reliability of the system. The mechanical joints are exposed to extreme temperature variations and a harsh environment which can lead to a connection failure that could be disastrous to the functioning of the superconducting device.